In orthopedic surgery it is common to rejoin broken bones. However, in most situations where cross-fixation of the bone fragments is required, the success of the surgical procedure depends to a large extent on the degree of compression that can be achieved between the bone fragments. More specifically, if the surgeon is unable to bring the bone fragments in close contact with each other, there will exist a space or void into which the bone tissue must grow before the healing process is complete. Thus, the greater the distance between the bone fragments, the slower the healing process. In addition, the healing process can be retarded by any relative movement or shifting of the bone fragments which disturbs the bone tissue that has been laid down.
Screws commonly used in fracture fixation are of the lag type, and comprise a threaded leading end and an enlarged head incorporating a means to engage a driving tool at the trailing end. Situations often arise in which the presence of a screw head has a deleterious effect on the outcome of the repair, specifically in cases where the screw must be inserted in or near a skeletal joint or where inadequate bone stock is available to allow countersinking of the screw head.
In response to the above-noted problems, attempts have been made to improve upon the lag type screw. An apparatus is described in U.S. Pat. No. 4,175,555 issued Nov. 27, 1979 to Timothy J. Herbert which discloses a bone screw capable of connecting two bone fragments under compression and which lacks a conventional screw head, allowing complete burial of the device in the bone substance. This is accomplished by a screw which is threaded at its leading and trailing ends. The threads are like-handed, but are of unequal pitch. More specifically, the pitch of the thread at the leading end exceeds the pitch at the trailing end, thus causing the leading end of the screw to advance at a slightly faster rate than the trailing end which causes the bone fragments to be brought under some degree of compression.
While somewhat effective, the Herbert screw is not without its inherent drawbacks. For example, as a result of the relatively small difference in the thread pitch between the leading and trailing threads, the screw has a very limited ability to draw together bone fragments that are not already intimately opposed upon screw insertion. In addition, since the pitch of the leading and trailing threads differs, the outside diameter of the leading threads must be smaller than the core or root diameter of the trailing threads to prevent binding. Thus, the leading end of the screw has a weaker purchase on the bone than does the trailing end as the force required to pull a screw out of its bore is a function of thread diameter.
An additional drawback of the Herbert screw is that it lacks the "feel" for tightness available to the surgeon in a conventional lag screw. When the Herbert screw is employed in a repair, the bore for the trailing threads is not tapped and can not be tapped because of thread synchronization problems. Therefore, the surgeon feels only the resistance due to the trailing threads penetrating the bone, thereby introducing the possibility of sub-optimal repairs.
It is accordingly an object of the present invention to provide a bone screw wherein a first and a second bone fragment can be connected under compression.
Another object of the invention is to provide a bone screw wherein a first and a second bone fragment can be connected under adjustable compression or adjustable distraction.
A further object of the invention is to provide a bone screw wherein thread pullout strength on either side of the fracture line is equalized.
A still further object of the invention is to provide a bone screw wherein the surgeon will have the "feel" of a conventional lag screw.
A still further object of the present invention is to provide a bone screw with adjustable compression that yields compressive force substantially equal to that of a conventional lag screw.
A still further object of the present invention is to provide a bone screw wherein the screw is completely buried within the bone.